The class D amplifier is an amplifier for power amplification by pulse width modulation of an input signal. It is extensively used as an audio amplifier for a mobile phone, for instance. This class D amplifier compares an input analog signal and its differential signal to a ramp carrier wave signal to deliver pulse-width-modulated positive-phase and negative-phase output pulse signals. As compared to a linear amplifier, the class D amplifier is featured by low power consumption because an output pulse signal may be driven by the pulsed voltage of low saturation.
The class D amplifier is disclosed in Patent Documents 1 and 2, for instance. FIG. 7 depicts a block diagram showing a formulation of a class D amplifier disclosed in Patent Document 1. In FIG. 7, a comparator for voltage feedback 140 compares a differential output signal of an integrating amplifier 124 to a ramp signal RAMP generated by a ramp signal generator 123. The differential output signal of the integrating amplifier 124 is obtained from a feedback signal as later described and an AUDIO IN input. A comparator for voltage feedback 142 compares a differential output signal of an integrating amplifier 126 to an inverted ramp signal RAMPB generated by the ramp signal generator 123. The differential output signal of the integrating amplifier 126 is obtained from another feedback signal as later described and the AUDIO IN input. The pulse width modulated pulse signal, representing the result of comparison by the comparator for voltage feedback 140, is amplified by a half bridge 128 and delivered to an output terminal OUTP. The pulse width modulated pulse signal, representing the result of comparison by the comparator for voltage feedback 142, is amplified by a half bridge 130 and delivered to an output terminal OUTN. A load 131 is connected across the output terminals OUTP and OUTN. The output terminal OUTP is connected to the integrating amplifier 124 via a resistor Rfb2 as a feedback path. A capacitor C102 is connected across the input and the output of the integrating amplifier 124. The output terminal OUTN is connected to the integrating amplifier 126 via a resistor Rfb1 as a feedback path. A capacitor C101 is connected across the input and the output of the integrating amplifier 126. The class D amplifier, formulated as described above, delivers a positive phase output pulse signal and a negative phase output pulse signal, pulse width modulated by a positive side circuit 121 and a negative side circuit 122, respectively, to either ends of the load 131.
FIG. 8 depicts a block diagram showing the constitution of a class D amplifier disclosed in Patent Document 2. In FIG. 8, the class D amplifier includes a voltage-controlled current source circuit F1 for converting an input voice voltage signal Sin to two currents of different polarities, and capacitive elements for integration C101, C102 in which charges of the signals converted to currents and charges of the feedback currents are stored. The class D amplifier also includes hysteresis comparators COMP1, COMP2 for conversion by PWM, and output buffers B1, B2 for amplifying outputs of the hysteresis comparators. These hysteresis comparators compare the potentials of the capacitive elements for integration C101, C102 to a reference voltage Vcom. The class D amplifier further includes output buffers B1 and B2 for amplifying the outputs of the hysteresis comparators and constant current source feedback circuits I1, I2 for feeding back the outputs of the output buffers B1 and B2, respectively.
The input signal Sin is converted to differential currents, inverted in polarity, and the feedback currents Ifb are added to the current of a current oscillator 2 by capacitors C1 and C2, and the resulting currents are compared to the reference voltage Vcom. By so doing, the pulse width modulated pulse signals are delivered to output terminals OUTP, OUTN. In the circuit constitution of FIG. 8, the current of the current oscillator is used to form a carrier, instead of using a ramp carrier signal. It should be noted that combining the difference between the differential currents and the carrier pulse currents by the capacitors and comparing the resulting currents with the reference voltage Vcom yields an effect similar to comparing the differential voltage with the ramp carrier voltage.    [Patent Document 1] U.S. Pat. No. 6,262,632    [Patent Document 2] JP Patent Kokai Publication No. JP-P2005-303814A